"Ist Dieser Platz Noch Frei?" by travelfrosch
OK, so now you found a beer hall or beer garden. Where do you sit? In general, it's open seating at large common tables. If a table's empty, it's usually OK to sit down. If there are open seats at an occupied table, ask the occupants if the empty chair(s) is/ are free (e.g. "Entschuldigung, sind Diese zwei Plätze noch frei?") Don't speak German? (or can't fake it like me?;) Be creative with a combination of simple English, gestures, and smiles. You'll generally be tolerated, if not welcomed.
Exceptions to the above: The most common exception to the common seating protocol is the "stammtisch." Tables with this marking (usually marked by a sign on the table or hanging above it) are reserved for "Stammgäste," or regular guests. Even most regular guests don't get to sit at these tables -- only those who are specifically invited. Most establishments have at least one stammtisch -- some (such as the Hofbräuhaus or the Augustiner Keller) have several.